1. Technical Field
Exemplary embodiments are directed to a beam generator and an apparatus for generating an aerial image using the same, and more particularly, to an extreme ultra-violet (EUV) beam generator and an apparatus for generating an aerial image for an exposure mask using the same.
2. Discussion of the Related Art
As semiconductor devices have become more highly integrated and downsized, finer patterns are required and wiring widths and gap spacings between the wirings need to be downsized. In particular, a photolithography exposure process for forming fine patterns becomes more challenging due to the higher resolution of the finer patterns, and thus improving the exposure process resolution has been the subject of considerable research. To improve the exposure process resolution, shortwave beams, such as an ultra-violet (UV) laser beam and an extreme ultra-violet (EUV) laser beam, have been used as the optical source for the exposure process.
Due to the high degree of integration and small critical dimension (CD) of recent semiconductor devices, tiny defects in the exposure mask can cause process failures in the circuit patterns transcribed onto on a silicon wafer by the exposure process. To detect tiny defects in the exposure mask, an aerial image of the exposure mask is usually generated in an apparatus for generating the aerial image (hereinafter, aerial image generating apparatus) before performing the exposure process, and the aerial image is fully inspected by a mask inspection process. In a conventional aerial image generating apparatus, the same shortwave beam as used for the exposure process is generated in the beam generator and is used as the optical source for generating the aerial image.
In particular, a conventional beam generator usually generates a shortwave beam the same as or similar to that of the exposure process by using a laser beam, such as an infrared laser beam, because a laser beam has a high degree of spatial and temporal coherence. For example, when the exposure process for patterning circuit patterns on the silicon wafer is performed using an EUV beam, the exposure mask should comprise materials sensitive to the EUV beam. As a result, the aerial image generating apparatus also uses the EUV beam as an incident beam for generating the aerial image for the exposure mask.
However, since the infrared laser beam is coherent with the EUV beam and the infrared laser beam tends to damage the aerial image generating apparatus, the EUV beam can be used without the infrared laser for generating the aerial image of the exposure mask. For those reasons, a conventional beam generator usually includes a beam splitter for separating the EUV beam and the infrared laser beams. The beam splitter reflects the EUV beam and transmits the infrared laser beams.
In a conventional beam generator for an aerial image generating apparatus, the reflectivity of the EUV beam from the beam splitter is low and thus the optical efficiency of the EUV beam is poor. When the incidence angle with respect to the beam splitter of coherent EUV beam and infrared laser beam is increased to increase the reflectivity of the EUV beam, the reflectivity of the infrared beam also increases. The increase of the reflectivity of the infrared beam strengthens the intensity of the infrared beam refracted into the beam splitter, thus heating the beam splitter. For those reasons, a conventional beam generator includes an additional cooling system for reducing the beam splitter temperature, which increases the maintenance and operational cost of the beam generator and of the aerial image generating apparatus including the beam generator.